Condenser microphone

ABSTRACT

There is provided a condenser microphone in which even if strong electromagnetic waves are applied from a cellular phone or the like, the balance between a filter circuit for a No. 2 pin on the hot side and a filter circuit for a No. 3 pin on the cold side is maintained. The condenser microphone includes a printed wiring board  200  housed in a microphone casing and a three-pin type output connector, and is configured so that a No. 1 pin of the output connector is connected directly to the microphone casing and is connected to a ground electrode of the printed wiring board  200  via a high-frequency choke coil IL; on the printed wiring board, a first filter circuit  401  connected to the No. 2 pin on the hot side and a second filter circuit  501  connected to the No. 3 pin on the cold side, both filter circuits each including capacitor devices C and inductor devices L, are mounted; and the first and second filter circuits  401  and  501  are disposed so as to be substantially symmetrical with each other with respect to an imaginary centerline X 1 -X 1.  In the condenser microphone, the high-frequency choke coil IL is provided in the number of two, and these choke coils IL 1  and IL 2,  together with the first and second filter circuits  401  and  501,  are disposed so as to be substantially symmetrical with each other with respect to the imaginary centerline X 1 -X 1.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on, and claims priority from, JapaneseApplication Ser. No. JP2009-261617, filed Nov. 17, 2009, the disclosureof which is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a condenser microphone and, moreparticularly, to a technique for preventing high-frequencyelectromagnetic waves which are generated from a cellular phone or thelike from intruding into a microphone casing.

BACKGROUND ART

A condenser microphone incorporates an impedance converter such as afield effect transistor (FET) or the like because the microphone unitthereof has a very high impedance. In the condenser microphone, aphantom power source is used, and microphone sound signals are sent viaa balanced shielded cable of the phantom power source.

To connect the balanced shielded cable, a three-pin type outputconnector is provided on the microphone casing (microphone grip in ahandheld microphone) side (for example, refer to Japanese PatentApplication Publication No. H11-341583). The output connector is aconnector specified in EIAJ RC-5236 “Latch Lock Type Round Connector forAudio Equipment”, and the configuration thereof is described below withreference to FIGS. 3 to 6.

FIG. 3 is a sectional view showing a state in which the output connectoris mounted in the microphone casing, and FIG. 4 is a front view of theoutput connector removed from the microphone casing. In FIG. 3, theoutput connector is shown in a cross section taken along the line A-A ofFIG. 4. FIG. 5 is a plan view of the output connector.

According to these figures, the output connector 10 includes adisc-shaped connector base 11 formed of an electrical insulatingmaterial such as PBT (polybutadiene terephthalate) resin. In theconnector base 11, three pins, that is, a No. 1 pin E for grounding, aNo. 2 pin SH on the hot side for signal, and a No. 3 pin SC on the coldside for signal are penetratingly provided by force fit, for example.

Concerning the handheld microphone, as shown in FIG. 3, the outputconnector 10 is mounted in a connector housing cylinder 20 screwed to anend portion of the microphone grip, not shown, formed in a cylindricalshape. Usually, the microphone grip including the connector housingcylinder 20 is formed of a metallic material such as brass, and alsofunctions as a casing for shielding the incorporated electrical parts.

In the connector base 11, a male screw 12 is provided to electricallyconnect the No. 1 pin E for grounding to the connector housing cylinder20. The male screw 12 is housed in a screw housing hole 13 pierced inthe radial direction in the connector base 11. Also, the connector base11 is provided with an earth terminal plate 14 having internal threads14 a threadedly engaging with the male screw 12 in the screw housinghole 13.

As shown in the plan view of FIG. 5, the earth terminal plate 14 and theNo. 1 pin E for grounding are electrically connected to each other via aconnecting member 15. As shown in FIG. 3, the male screw 12 is broughtinto contact with the peripheral edge of a round hole 21, which ispierced in the connector housing cylinder 20, by turning the male screw12 using a screwdriver, not shown, from the round hole 21.

Thereby, the No. 1 pin E for grounding and the connector housingcylinder 20 are electrically connected to each other via the male screw12, the earth terminal plate 14, and the connecting member 15. Inaddition to this configuration, as shown in FIGS. 4 and 5, in somecases, a plate spring 16 contacting with the inner surface of theconnector housing cylinder 20 is connected to the No. 1 pin E forgrounding so that the No. 1 pin E for grounding and the connectorhousing cylinder 20 are electrically connected to each other by thisplate spring 16.

Even if the No. 1 pin E for grounding is electrically connected to theconnector housing cylinder 20 in this manner, when strongelectromagnetic waves emitted from a cellular phone or the like areapplied to the microphone or a microphone cable (balanced shieldedcable) in the state in which the microphone cable pulled out of thephantom power source (not shown) side is connected to the outputconnector 10, in some cases, the electromagnetic waves intrude into themicrophone through the output connector 10, being demodulated by theimpedance converter, and are delivered from the microphone as noisehaving an audio frequency.

To solve this problem, the invention described in Japanese Patent No.4273019 has disclosed a condenser microphone in which an electroniccircuit for microphone unit is incorporated in the microphone casing,and the output connector is mounted to an end portion of the microphonecasing. In this condenser microphone, the No. 1 pin for grounding isconnected directly to the microphone casing and is connected to theground of the electronic circuit via a high-frequency choke coil, andthe ground is connected to the microphone casing via a lead wire,whereby high-frequency electromagnetic waves, which may cause thegeneration of noise, are prevented from intruding into the microphonecasing through the output connector.

As another method for preventing electromagnetic waves from intrudinginto the microphone through the output connector 10, there is availablea method in which, as shown in FIG. 5, capacitor devices C areconnectingly provided between the No. 1 pin E for grounding and the No.2 pin SH on the hot side and between the No. 1 pin E for grounding andthe No. 3 pin SC on the cold side, and the No. 2 pin SH on the hot sideand the No. 3 pin SC on the cold side are connected to the microphonecasing including the connector housing cylinder 20 via an inductordevice L for inhibiting high frequencies from intruding.

By a filter circuit consisting of the capacitor devices C and theinductor devices L, ordinary broadcast waves and electromagnetic wavessuch as HF, VHF, and UHF waves can be prevented from intruding almostwithout problems. However, a problem occurring when the condensermicrophone is exposed to considerably strong electromagnetic wavesemitted from a cellular phone is described below with reference to aschematic view of FIG. 6.

In FIG. 6, Z1 denotes an impedance of the filter circuit including thecapacitor devices C and the inductor devices L, which are connected tothe No. 2 pin SH on the hot side on the primary side of an outputtransformer T of the microphone, Z2 denotes an impedance of the filtercircuit including the capacitor devices C and the inductor devices L,which are connected to the No. 3 pin SC on the cold side similarly onthe primary side of an output transformer T of the microphone, Z3 and Z4are impedances existing on the hot side and the cold side, respectively,of the balanced shielded cable for phantom power source. In order fornoise to be canceled, design is made so that the impedances Z1 and Z2are in balance, and the impedances Z3 and Z4 are in balance.

However, if the impedances Z1 and Z2 of each filter circuit are not inbalance (imbalanced) in a high frequency region, when a high-frequencysignal Ua is fed from a cellular phone or the like, an imbalanced signalUb that is not canceled is generated between the hot side and the coldside, whereby a high-frequency current is caused to flow in themicrophone casing including the connector housing cylinder 20, so thatnoise louder than ordinary is generated.

In the case where the output transformer T of the microphone is wired byusing a relatively long lead wire, depending on the length and layout ofthe lead wire, the impedances Z1 and Z2 of each filter circuit becomeimbalanced even at frequencies of VHF band, so that noise may begenerated.

Considering this issue, in the invention described in Japanese PatentApplication Publication No. 2007-324804, as shown in FIG. 7, in forminga first filter circuit 102 for the No. 2 pin on the hot side and asecond filter circuit 103 for the No. 3 pin on the cold side on aprinted wiring board 100, both of the filter circuits 102 and 103 aresubstantially symmetrical (in FIG. 7, symmetrical in the up-and-downdirection) with each other with respect to the X-X line passing througha middle-point line pattern 143, for example, connected to amiddle-point tap of the output transformer.

That is, the first filter circuit 102 includes three land parts 121, 122and 123, a common mode choke coil CL, two capacitor devices C1 and C2,and one inductor device L1. Similarly, the second filter circuit 103includes three land parts 131, 132 and 133, a common mode choke coil CL,two capacitor devices C1 and C2, and one inductor device L1. These landparts and devices are arranged so as to be substantially symmetricalwith each other, respectively, with respect to the X-X line.

Also, a No. 1 pin connection land 110 to which the No. 1 pin forgrounding is connected and a No. 2 pin connection land 120 to which theNo. 2 pin on the hot side is connected are arranged so as to besubstantially symmetrical with each other with respect to the X-X line.Because of the space condition, the No. 3 pin on the cold side isconnected to a No. 3 pin connection land on the back surface side of theprinted wiring board 100, and the No. 3 pin connection land is pulledout to the top surface side of the printed wiring board 100 via athrough hole wiring 130.

Also, according to the invention described in Japanese Patent No.4273019, a high-frequency choke coil IL is connectingly provided betweenthe No. 1 pin connection land 110 and a ground electrode 101. Althoughnot shown in the figure, the No. 1 pin is connected directly to themicrophone casing.

According the invention described in Japanese Patent ApplicationPublication No. 2007-324804, the first filter circuit 102 for the No. 2pin on the hot side and the second filter circuit 103 for the No. 3 pinon the cold side, including the printed circuit patterns mounting thesefilter circuits, are arranged so as to be substantially symmetrical witheach other with respect to the middle point line pattern 143, forexample, connected to the middle-point tap of the output transformer.Therefore, since the equilibrium of microphone output is maintained upto a high frequency region, even if the microphone is exposed to stronghigh-frequency electromagnetic waves emitted from a cellular phone orthe like, a high-frequency current does not flow in the microphonecasing, so that noise can be prevented from being generated byextraneous noise.

Unfortunately, according to invention described in Japanese PatentApplication Publication No. 2007-324804, the high-frequency choke coilIL connectingly provided between the No. 1 pin for grounding and theground electrode 101 is arranged on one side of the filter circuits 102and 103, for example, so as to be imbalancedly on the first filtercircuit 102 side in the example shown in FIG. 7, so that a problemdescribed below is pointed out.

The high-frequency choke coil IL generates a high-frequency magneticfield on account of the feed of high-frequency signals, and ismagnetically connected to the inductors (L1 and CL) on the signal side.This magnetic connection (mutual induction) depends on the distancebetween the parts.

Therefore, high mutual induction is generated between the high-frequencychoke coil IL and the inductor close to the high-frequency choke coil IL(for example, the inductor device L1 in the first filter circuit 102),and in contrast, low mutual induction is generated between thehigh-frequency choke coil IL and the inductor far from thehigh-frequency choke coil IL (for example, the inductor device L1 in thesecond filter circuit 103).

For this reason, although the filter circuits 102 and 103 are arrangedsymmetrically, the symmetry of the high-frequency current collapses,whereby the high-frequency current is allowed to intrude into themicrophone casing, so that noise is still generated in some cases.

Accordingly, an object of the present invention is to provide acondenser microphone in which a filter circuit for the No. 2 pin on thehot side and a filter circuit for the No. 3 pin on the cold side arearranged symmetrically on a printed wiring board incorporated in amicrophone casing, and a high-frequency choke coil is provided toconnect the No. 1 pin for grounding to a ground electrode, wherein ahigh-frequency magnetic field generated from the high-frequency chokecoil is applied evenly to inductors included in the filter circuits.

SUMMARY OF THE INVENTION

To achieve the above object, the present invention provides a condensermicrophone including a microphone casing formed of a metallic material;a printed wiring board that has a sound signal output circuit connectedto a condenser microphone unit and is housed in the microphone casing;and an output connector which includes a No. 1 pin for grounding, a No.2 pin on the hot side for signal, and a No. 3 pin on the cold sidetherefor and is mounted in an end portion of the microphone casing, inwhich the No. 1 pin is connected directly to the microphone casing andis connected to a ground electrode of the printed wiring board via ahigh-frequency choke coil; on the printed wiring board, a first filtercircuit connected to the No. 2 pin on the hot side and a second filtercircuit connected to the No. 3 pin on the cold side, both filtercircuits each including capacitor devices and inductor devices, aremounted; and the first filter circuit and the second filter circuit,including printed circuit patterns for mounting these filter circuits,are disposed so as to be substantially symmetrical with each other withrespect to an imaginary centerline, wherein the high-frequency chokecoil connectingly provided between the No. 1 pin and the groundelectrode is provided in the number of two, and these choke coils,together with the first and second filter circuits, are disposed so asto be substantially symmetrical with each other with respect to theimaginary centerline.

According to a preferred mode of the present invention, the imaginarycenterline is the centerline of the printed wiring board, which dividesthe printed wiring board into two parts evenly.

In the case where the sound signal output circuit includes an outputtransformer, the imaginary centerline is a middle-point line patternconnected to a middle-point tap of the output transformer.

According to the present invention, the high-frequency choke coilsconnectingly provided between the No. 1 pin for grounding and the groundelectrode, together with the first filter circuit connected to the No. 2pin on the hot side and the second filter circuit connected to the No. 3pin on the cold side, are disposed so as to be substantially symmetricalwith each other with respect to the imaginary centerline. Thereby,high-frequency magnetic fields generated by the high-frequency chokecoils are applied evenly to the inductors included in the filtercircuits, and mutual induction generated therebetween becomes almostequal.

Therefore, the balance is maintained in terms of high frequency, and ahigh-frequency current caused by strong extraneous electromagnetic wavesemitted from a cellular phone or the like can be inhibited fromintruding into the microphone casing with high reliability, so that acondenser microphone having high resistance to noise is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a filter circuit formed on the top surface sideof a printed wiring board that a condenser microphone in accordance withan embodiment of the present invention has;

FIG. 2 is a plan view showing the back surface side of the printedwiring board shown in FIG. 1;

FIG. 3 is a sectional view showing a state in which a three-pin typeoutput connector is mounted in a connector housing cylinder;

FIG. 4 is a front view of the output connector shown in FIG. 3;

FIG. 5 is a plan view of the output connector shown in FIG. 3;

FIG. 6 is a schematic view of an output equilibrium circuit of acondenser microphone; and

FIG. 7 is an enlarged plan view of filter circuits of the inventiondescribed in Japanese Patent Application Publication No. 2007-324804.

DETAILED DESCRIPTION

An embodiment of the present invention will now be described withreference to FIGS. 1 and 2. The present invention is not limited to thisembodiment. In the description of this embodiment, since a condensermicrophone unit may have a publicly known configuration, theillustration thereof is omitted, and also, as for an output connector,since the before-described three-pin type output connector 10 can beused as it is, refer to FIGS. 3 to 5 as appropriate.

FIG. 1 shows the top surface 201 side of a printed wiring board 200 ofthis embodiment, and FIG. 2 shows the back surface 202 side thereof. Theprinted wiring board 200 is a wiring board housed in a microphonecasing, not shown, formed of a metallic material such as brass havingelectric conductivity. Although not shown, on the left-hand side in FIG.1 of the printed wiring board 200, a sound signal output circuit forcondenser microphone unit is provided.

In the sound signal output circuit, a PAD circuit, a field effecttransistor (FET) serving as an impedance converter, an outputtransformer, and the like are mounted. Also, on the right-hand side inFIG. 1 of the printed wiring board 200, the output connector 10 isdisposed, and pins (No. 1 pin, No. 2 pin, No. 3 pin) are connected topredetermined portions of the printed wiring board 200 directly or vialead wires, not shown.

For the connection with the output connector 10, the printed wiringboard 200 includes a No. 1 pin connection land 300 to which the No. 1pin for grounding is connected, a No. 2 pin connection land 400 to whichthe No. 2 pin on the hot side is connected, and a No. 3 pin connectionland 500 to which the No. 3 pin on the cold side is connected.

In this embodiment, for convenience of lead wiring, wiring board space,and the like, the No. 1 pin connection land 300 and the No. 2 pinconnection land 400 are disposed on the top surface 201 side of theprinted wiring board 200. In contrast, as shown in FIG. 2, the No. 3 pinconnection land 500 is disposed on the back surface 202 side of theprinted wiring board 200, and is pulled out to the top surface 201 sidevia a through hole wiring TH3.

In forming a first filter circuit 401 for the No. 2 pin and a secondfilter circuit 501 for the No. 3 pin on the top surface 201 side of theprinted wiring board 200, the top surface 201 of the printed wiringboard 200 is partitioned into a first mount region 203 on the upper sidein FIG. 1 and a second mount region 204 on the lower side therein by animaginary centerline X1-X1.

The centerline X1-X1 may be a wiring board centerline that divides theprinted wiring board 200 into two parts evenly. Alternatively, in thecase where an output transformer exists in the sound signal outputcircuit, a middle-point line pattern connected to a middle-point tap ofthe output transformer may be made the centerline X1-X1. In thisembodiment, the centerline X1-X1 is the wiring board centerline.

In this embodiment, the No. 1 pin connection land 300 is disposed on thesecond mount region 204 side; however, on the first mount region 203side as well, a No. 1 pin pulling-out electrode 320 electricallyconnected to the No. 1 pin connection land 300 is provided.

Referring to FIG. 2, on the back surface 202 side of the printed wiringboard 200, a connection electrode 310 is formed in an overall patternshape along the width direction (the up-and-down direction in FIG. 2) ofthe wiring board, and the No. 1 pin connection land 300 and the No. 1pin pulling-out electrode 320 are electrically connected to each othervia through hole wirings TH1 and TH2 and the connection electrode 310.

Also, as shown in FIG. 2, in the remaining portion (a portion other thanthe No. 3 pin connection land 500 and the connection electrode 310) ofthe back surface 202 of the printed wiring board 200, a ground electrode210 is formed in an overall pattern shape.

Referring to FIG. 1, in the first mount region 203, as a device mountingpattern of the first filter circuit 401 for the No. 2 pin, there areprovided a branch electrode 321 connecting with the No. 1 pinpulling-out electrode 320, a branch electrode 410 connecting with theNo. 2 pin connection land 400, a first land part 420 and a second landpart 430 each formed in an island shape, and an electrode terminal pad221 formed in an end portion of a signal line 220 leading to oneterminal of the sound signal output circuit.

In FIG. 1, taking the lengthwise direction (the right-and-leftdirection) of the printed wiring board 200 as the transverse directionand the width direction (the up-and-down direction) thereof as thelongitudinal direction, the branch electrode 321 is pulled out of theleft end of the No. 1 pin pulling-out electrode 320 toward the leftdirection.

The branch electrode 410 has a first electrode part 411 pulled out ofthe left end lower side of the No. 2 pin connection land 400 toward theleft direction and a second electrode part 412 pulled out of the firstelectrode part 411 in the left lower slantwise direction toward thecenterline X1-X1 side.

The first land part 420 is disposed as a whole in a substantiallycentral portion of the first mount region 203, and has a trunk electrodepart 421 formed in the longitudinal direction, a first electrode part422 pulled out of the upper end of the trunk electrode part 421 towardthe left direction, and a second electrode part 423 and a thirdelectrode part 424 pulled out of the lower end side of the trunkelectrode part 421 toward the right direction so as to be parallel witheach other and in a fork shape.

The trunk electrode part 421 is electrically connected to the groundelectrode 210 lying on the back surface 202 of the printed wiring board200 via through hole wirings TH4.

The first electrode part 422 of the first land part 420 faces theelectrode terminal pad 221 of the signal line 220 with a predeterminedspace provided therebetween, the second electrode part 423 faces thebranch electrode 321 with a predetermined space provided therebetween,and a part of the third electrode part 424 faces the first electrodepart 411 of the branch electrode 410 with a predetermined space providedtherebetween.

The second land part 430 is disposed in the left lower corner of thefirst mount region 203, and has a first electrode part 431 extending inthe transverse direction and a second electrode part 432 extending fromthe left end of the first electrode part 431 to the lower side in thelongitudinal direction.

The first electrode part 431 faces the electrode terminal pad 221 of thesignal line 220 and the third electrode part 424 of the first land part420 with a predetermined space provided therebetween, and the secondelectrode part 432 faces the second electrode part 412 of the branchelectrode 410 with a predetermined space provided therebetween.

In the first mount region 203, there are mounted a harmonic choke coilIL1 constituting the first filter circuit 401 for the No. 2 pin, acommon mode choke coil CL1, three capacitor devices C41 to C43, and aninductor device L41. All of these parts are preferably chip parts.

The harmonic choke coil IL1 is connectingly provided between the branchelectrode 321 of the No. 1 pin pulling-out electrode 320 and the secondelectrode part 423 of the first land part 420. The common mode chokecoil CL1 is connectingly provided between the second electrode part 412of the No. 2 pin connection land 400 and the second electrode part 432of the second land part 430.

The capacitor device C41 is connectingly provided between the thirdelectrode part 424 of the first land part 420 and the first electrodepart 411 of the No. 2 pin connection land 400. The capacitor device C42is connectingly provided between the third electrode part 424 of thefirst land part 420 and the first electrode part 431 of the second landpart 430.

The capacitor device C43 is connectingly provided between the firstelectrode part 422 of the first land part 420 and the electrode terminalpad 221 of the signal line 220. Also, the inductor device L41 isconnectingly provided between the first electrode part 431 of the secondland part 430 and the electrode terminal pad 221 of the signal line 220.

In the second mount region 204, as a device mounting pattern of thesecond filter circuit 501 for the No. 3 pin, there are provided a branchelectrode 301 connecting with the No. 1 pin connection land 300, a No. 3pin pulling-out electrode 510 connecting with the No. 3 pin connectionland 500, a first land part 520 and a second land part 530 each formedin an island shape, and an electrode terminal pad 231 formed in an endportion of a signal line 230 leading to the other terminal of the soundsignal output circuit.

The branch electrode 301 is pulled out of the left end of the No. 1 pinconnection land 300 toward the left direction, and is disposed so as tobe symmetrical with the branch electrode 321 with respect to thecenterline X1-X1.

The No. 3 pin pulling-out electrode 510 is pulled out of the No. 3 pinconnection land 500 on the back surface 202 side to the top surface 201side via the through hole wiring TH3, and the end portion thereof isprovided with a first electrode part 511. The first electrode part 511is disposed so as to be symmetrical with the second electrode part 412with respect to the centerline X1-X1.

The first land part 520 is formed as a pattern symmetrical with thefirst land part 420 with respect to the centerline X1-X1.

That is to say, the first land part 520 has a trunk electrode part 521formed in the longitudinal direction, a first electrode part 522 pulledout of the lower end of the trunk electrode part 521 toward the leftdirection, and a second electrode part 523 and a third electrode part524 pulled out of the upper end side of the trunk electrode part 521toward the right direction so as to be parallel with each other and in afork shape.

The first electrode part 522 corresponds to the first electrode part 422of the first land part 420, and the second electrode part 523 and thethird electrode part 524 correspond to the second electrode part 423 andthe third electrode part 424, respectively, of the first land part 420.

The trunk electrode part 521 is electrically connected to the groundelectrode 210 lying on the back surface 202 side of the printed wiringboard 200 via through hole wirings TH5.

The first electrode part 522 of the first land part 520 faces theelectrode terminal pad 231 of the signal line 230 with a predeterminedspace being provided therebetween, the second electrode part 523 facesthe branch electrode 301 with a predetermined space being providedtherebetween, and a part of the third electrode part 524 faces the No. 3pin pulling-out electrode 510 with a predetermined space being providedtherebetween.

The second land part 530 is formed as a pattern symmetrical with thesecond land part 430 with respect to the centerline X1-X1. That is, thesecond land part 530 has a first electrode part 531 extending in thetransverse direction and a second electrode part 532 extending from theleft end of the first electrode part 531 to the upper side in thelongitudinal direction.

The first electrode part 531 faces the electrode terminal pad 231 of thesignal line 230 and the third electrode part 524 of the first land part520 with a predetermined space provided therebetween, and the secondelectrode part 532 faces the first electrode part 511 of the No. 3 pinpulling-out electrode 510 with a predetermined space providedtherebetween.

In the second mount region 204, there are mounted a harmonic choke coilIL2 constituting the second filter circuit 501 for No. 3 pin, a commonmode choke coil CL2, three capacitor devices C51 to C53, and an inductordevice L51. As all of these parts, parts that are the same as the partsmounted in the first mount region 203 are used.

The harmonic choke coil IL2 is connectingly provided between the branchelectrode 301 of the No. 1 pin land 300 and the second electrode part523 of the first land part 520. The common mode choke coil CL2 isconnectingly provided between the first electrode part 511 of the No. 3pin pulling-out electrode 510 and the second electrode part 532 of thesecond land part 530.

The capacitor device C51 is connectingly provided between the thirdelectrode part 524 of the first land part 520 and the No. 3 pinpulling-out electrode 510. The capacitor device C52 is connectinglyprovided between the third electrode part 524 of the first land part 520and the first electrode part 531 of the second land part 530.

The capacitor device C53 is connectingly provided between the firstelectrode part 522 of the first land part 520 and the electrode terminalpad 231 of the signal line 230. Also, the inductor device L51 isconnectingly provided between the first electrode part 531 of the secondland part 530 and the electrode terminal pad 231 of the signal line 230.

Thus, the harmonic choke coils IL1 and IL2, the common mode choke coilsCL1 and CL2, the capacitor devices C41 and C51, the capacitor devicesC42 and C52, the capacitor devices C43 and C53, and the inductor devicesL41 and L51 are disposed so as to be symmetrical with each other,respectively, with respect to the centerline X1-X1.

In particular, since the harmonic choke coils IL1 and IL2 are disposedso as to be symmetrical with each other in the first mount region 203and the second mount region 204, high-frequency magnetic fieldsgenerated by the harmonic choke coils IL1 and IL2 are applied evenly tothe inductors (CL1, CL2, L41, L51) included in the filter circuits 401and 501, and mutual induction generated therebetween becomes almostequal. Therefore, the balance is maintained in terms of high frequency,and a high-frequency current caused by strong extraneous electromagneticwaves emitted from a cellular phone or the like can be inhibited fromintruding into the microphone casing with high reliability, so that acondenser microphone having high resistance to noise is provided.

1. A condenser microphone comprising: a microphone casing formed of ametallic material; a printed wiring board which has a sound signaloutput circuit connected to a condenser microphone unit, and is housedin the microphone casing; and an output connector which includes a No. 1pin for grounding, a No. 2 pin on the hot side for signal, and a No. 3pin on the cold side therefor, and is mounted in an end portion of themicrophone casing, in which the No. 1 pin is connected directly to themicrophone casing and is connected to a ground electrode of the printedwiring board via a high-frequency choke coil; on the printed wiringboard, a first filter circuit connected to the No. 2 pin on the hot sideand a second filter circuit connected to the No. 3 pin on the cold side,both filter circuits each including a capacitor device and an inductordevice, are mounted; and the first filter circuit and the second filtercircuit, including printed circuit patterns for mounting these filtercircuits, are disposed so as to be substantially symmetrical with eachother with respect to an imaginary centerline, wherein thehigh-frequency choke coil connectingly provided between the No. 1 pinand the ground electrode is provided in the number of two, and thesechoke coils, together with the first and second filter circuits, aredisposed so as to be substantially symmetrical with each other withrespect to the imaginary centerline.
 2. The condenser microphoneaccording to claim 1, wherein the imaginary centerline is the centerlineof the printed wiring board, which divides the printed wiring board intotwo parts evenly.
 3. The condenser microphone according to claim 1,wherein the sound signal output circuit includes an output transformer,and the imaginary centerline is a middle-point line pattern connected toa middle-point tap of the output transformer.